The present application is a U.S. non-provisional application based upon and claiming priority from Japanese Application No. HEI 10-374458, which is hereby incorporated by reference.
The present invention relates to a continuous polycarbonate manufacturing method in which a polycarbonate with few admixtures is manufactured continuously, stably, and efficiently.
Polycarbonates have excellent mechanical properties such as impact resistance, as well as excellent heat resistance, transparency and other properties. They are widely used in applications such as various types of mechanical components, optical disks, and automotive parts. They are particularly promising for optical applications such as memory-use optical disks, optical fibers, and lenses.
Known methods for manufacturing these polycarbonates include a method in which a bisphenol such as bisphenol A is allowed to react directly with phosgene (interfacial method), and a method in which a bisphenol such as bisphenol A is subjected to a melt polycondensation reaction (transesterification reaction) with a carbonic diester such as diphenyl carbonate.
Of these two, the interfacial method using phosgene is the more commonly implemented. On the other hand, an advantage of the transesterification method is that a polycarbonate can be manufactured more inexpensively than with the interfacial method, and because transesterification does not involve the use of a toxic substance such as phosgene, it is very promising as a polycarbonate manufacturing method.
Still, if the manufacture of a polycarbonate is carried out continuously over an extended period by this transesterification method, white foreign material can become admixed in the manufactured polycarbonate and can clog the piping lines, thereby lowering the manufacturing efficiency.
As a result of diligent research conducted in light of these problems, the inventors discovered that the lower polycarbonate polycondensate produced in the intermediate stage of a polycondensation reaction can crystallize when heated and become a source of white foreign material, and can crystallize on the pipe surfaces and become a cause of pipe clogging.
Upon further research, the inventors arrived at the present invention upon discovering that a lower polycarbonate polycondensate having an intrinsic viscosity between 0.1 and 0.4 dL/g readily undergoes crystallization at temperatures below 230.degree. C., and therefore found that if the polycondensation of a polycarbonate is carried out by setting the temperature to be at least 230.degree. C. on the surface of the reaction equipment in direct contact with a lower polycarbonate polycondensate having an intrinsic viscosity between 0.1 and 0.4 dL/g, then the admixture of white foreign material and the clogging of the piping due to polycarbonate crystallization will be suppressed, and a polycarbonate with excellent hue stability will be obtained efficiently.